Neuromuscular disease, chronic obstructive pulmonary disease (COPD) and obese hypoventilation patients often suffer from chronic respiratory failure. Said patients need regular treatment of their respiratory failure at home. Hypoxemic patients are treated by oxygen therapy (mostly without ventilator support), while treatment by Invasive Ventilation (IV) and Non Invasive Ventilation (NIV) with environmental air helps bringing the high carbon dioxide (CO2) blood gas level of hypercapnic patients back to an acceptable level. The efficacy of the ventilation is checked by measuring the base-line and the trends in the arterial oxygen and carbon dioxide levels during nocturnal NIV.
Arterial blood gas measurements form the golden standard. Before starting ventilation treatment at home, patients stay at the hospital to optimize ventilator settings and monitor arterial blood gas values. Depending on disease severity and stability, patients have to return more or less regularly to the hospital for checks. A respiratory nurse can also visit the patient at home to check the ventilator and to install equipment that enables non-invasive monitoring of blood gas partial pressures. At home, blood gas levels are monitored typically during a night and data are stored together with ventilator and respiratory data for later analysis at the hospital.
The state of the art in non-invasive blood oxygenation monitoring, is by measuring the arterial oxygen saturation, which relates to the partial oxygen pressure via the oxygen dissociation curve. Pulse oximetry (SpO2) is an optical method for non-invasive monitoring of arterial oxygen saturation in a patient and has become one of the most commonly used technologies in clinical practice. Pulse oximetry is a reasonably low cost technology and is easy to use. It is the preferred method for blood oxygenation monitoring at home.
The state of the art in non-invasive monitoring of the partial pressure of CO2 is by means of capnography or by transcutaneous CO2 (PtcCO2) monitoring. For intubated patients with a healthy lung the end tidal CO2 (etCO2) value obtained by capnography offers a good indication of the arterial CO2 value. However, in case of non-invasive ventilation where air leaks between mask and face are usually present and the patients have severe respiratory diseases capnography is often not a reliable method. In most hospitals a combination is used of capnography for trend monitoring and analysis of an arterial blood sample to obtain an occasional accurate value.
Transcutaneous CO2 monitoring is not disrupted by air-leaks and respiratory diseases but requires trained personal to obtain reliable values and shows some inaccuracy due to variation in skin properties among adults. At home CO2 blood gas monitoring is less frequently used than oximetry despite its high relevance for patients receiving ventilation.
Current transcutaneous CO2 sensors are all based on a 40 year old concept of (i) a thermostatically controlled heater to increase blood perfusion and gas-permeability of the skin; (ii) a fluid layer between skin and sensor membrane; (iii) a gas-permeable membrane covering the sensor; (iv) an electrolyte solution between membrane and sensor; (v) a sensor comprising an electrochemical pH sensor and reference electrode; and (v) an algorithm to compensate for temperature effects and skin metabolism.
U.S. Pat. No. 6,602,716 B1 describes a method and device for fluorimetric determination of a biological, chemical or physical parameter, in particular gaseous CO2 or NH3, of a sample utilizing two different luminescent materials, the first being sensitive to the parameter, at least with respect to the luminescence intensity, the second being insensitive to the parameter, at least with respect to luminescence intensity and decay times. The application further indicates that phase modulation techniques can be employed to determine the mean phase shift of the luminescence signal. The phase angle φm thus depends on the ratio of the two signal intensities but not on the absolute signal level and will permit the referencing of the intensity of the short-lived indicator dye component.
A further example of a prior art chemo-optical sensor for transcutaneous application is depicted in FIG. 1, wherein on top of an optical transparent carrier material two layers of “silicon rubber-like” gas-permeable materials are deposited The first layer—the sensing layer—comprises a mixture of two luminescent dyes in a lipophilic phase transfer agent within a hydrophobic polymer, namely a reference dye having a long luminescent life-time and a pH-sensitive indicator dye having a short luminescent life-time. A second membrane layer comprises light reflecting material (TiO2) particles and prevents ion transport to and from the sensing layer. CO2 gas typically diffuses through said membrane into the first (sensing) layer and changes the pH, which in turn modifies the fluorescence from the indicator dye. By using a dual life-time referencing technique, which effectively measures the time response of modulated light excitation, the percentage of CO2 gas can be calculated.
The lipophilic phase transfer agent also serves as chemical buffer material to provide water for the production of carbonic acid. However, osmotic imbalance at the site of application of the sensor may initiate, for example, water transport in the sensor, which could lead to unwanted sensitivity changes of the sensor. Typically, such changes require a complete and time-consuming re-calibration of the sensor.
In consequence, there is a need for the development of a methodology allowing to compensate induced gas concentration measurement inaccuracy, in particular without the need of additional re-calibration of the chemo-optical sensor.